An audit launched last Tuesday revealed a Colt M16 assault rifle has gone missing from a gun vault at the Philadelphia Police Training Academy in Northeast Philadelphia, Police Commissioner Charles Ramsey announced Monday at a press conference.

"It was either taken or there was an inventory error to begin with," Ramsey said.

The Internal Affairs Unit, together with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, has launched a full investigation into the incident.

The M16 was one of nearly 1,400 surplus weapons the Department received from the military in August 2009.

"It is department property – we were entrusted with this, it was given to us," Ramsey said.

An inventory of the firearms was last completed in December, according to the commissioner.

"Initially when we got [the audit] in October, they were taken out of the crates, they were individually counted and the serial numbers were matched," he said.

"That's what I've been told. So if that's the case then we had 1,356. Yet last week when they were taken out of the crate and they were counted, it was 1,355."

He said investigators are searching for the missing machine gun, conducting departmental audits and inspecting all police districts, special units and lockers.

The police Office of Audit and Compliance is undertaking a department-wide audit of every firearm in the police department.

"We just became aware of this," Ramsey said.

"I was hoping that we would locate it over the weekend. It is possible – I mean, one weapon out of 1,356 – that it could certainly be somewhere or it was issued and someone didn't do the proper paperwork or whatever. So we're still matching serial numbers in different units and so forth to see if we can come up with it."

But police have had no luck so far.

The M16s are secured on pallets with metal strap bands and stored in an 800-pound crate inside a locker equipped with an alarm and protected by both a deadbolt and a keypad lock.

Ramsey said only a few people have access to the code.

"This was not someone who came in from the outside to take this," he said.

"There's no indication of that at all. So I can almost say the odds of that being on the street are slim and none, that this is not that type of issue. But the fact is it shouldn't be anywhere other than in that vault, period."

Ramsey said it's unclear whether the loss of the weapon resulted from an bookkeeping error or if it was taken by an officer without the department's permission.

"It could be an inventory issue, although we're going through everything we can and so far we have still not located it," he said.

"My biggest fear, obviously, is that it was stolen by one of my own members."

Ramsey anticipates reforms in how gun storage facilities are secured in the future, starting with changing the locks at the academy facility.

"There's no video trained on that door, that's part of it," he said."That we're going to correct and make sure that we have that. But that's a day late and a dollar short. That's for the future."

Officials at the armory were in the process of converting the M16s, which can be fired automatically or semiautomatically, to AR-15s, which are semiautomatic only.

Ramsey believes the missing gun had not yet been converted.

"We're cross-training some of our officers for active shooter training and the like," he said."That's why we have these types of weapons – to supplement what our SWAT teams use, should we have a situation where we need to contain a suspect or we have something, unfortunately, going on inside of a building or something like that.

"That's why we have them. That's why we asked for them. But we also have the responsibility to make sure that we maintain them and that we keep track of them."

He said if it turns out the weapon was stolen, he plans to bring down the full force of the law on the person responsible.

"A federal offense is a federal offense," he said.

"And I guarantee you that if somebody that I find out took that gun, I'll do everything in my power to see to it they get time in a federal penitentiary."

As far as the exact charge any suspect would face, Ramsey said, "I don't know what it is, but whatever the hell it is, we're going to use it."